The present invention relates to textured heat transfer surfaces. More particularly, the present invention relates to finstock which is micro-textured to provide increased surface area and increased turbulence of air flowing thereover, and to tubestock, turbulators and the like which are micro-textured to provide increased surface area and increased coolant or refrigerant flow thereover.
Aluminum and its alloys are particularly useful materials for heat exchangers in a variety of applications including vehicles such as cars, trucks, airplanes, and the like. Aluminum alloys are lighter than steel alloys and thus offer weight advantages in many applications in vehicles. The light weight and excellent heat transfer properties of aluminum alloys make them particularly attractive candidates for use in heat exchangers such as radiators, heaters, evaporators, oil coolers, condensers and the like. These heat exchangers and similar components are typically fabricated from mill finished brazing sheet which may be clad or unclad. Conventional aluminum brazing sheet typically includes two to four roll bonded layers with at least one of the exterior layers being an Aluminum Association (AA) 4xxx series alloy and the other layers being 1xxx, 2xxx, 3xxx, 4xxx, 5xxx, 6xxx, 7xxx or 8xxx series alloy. Mill finished brazing sheet has an inherent roughness of about 0.7 micron root mean squared (RMS) or less. All dimensions referred to hereinafter include the RMS value thereof.
Aluminum brazing sheet is fabricated into the tubes of heat exchangers and the fins attached thereto. The efficiency of a heat exchanger is significantly affected by many variables including the total surface area of the heat transfer surfaces and the persistence of thermal boundary layers on the heat transfer surfaces. Hence, it is generally desirable to maximize the size of the heat transfer surface area and to turbulate the heat transfer media (coolant, air, refrigerant, etc.) to disrupt the boundary layer and maximize heat transfer.
For finstock, turbulators and the like, one approach to enhancing turbulation of the heat transfer medium has been to use louvers which are members that are mechanically flared out into the air or coolant stream, and cause re-direction (i.e. turbulation) of the air or coolant. Conventional louvers on finstock are about 0.5-2 millimeters (mm) high and are spaced apart by about 1 mm. The length of a louver typically is about 80-90 percent of the length of a fin. Other types of embossments for heat exchanger fins are disclosed in U.S. Pat. Nos. 4,434,846 and 4,984,626. Louvers and other embossments must be incorporated into heat exchanger components with due regard for the geometry of the components. Due to their size and configuration, louvers and embossments can only be used on a limited variety of heat exchanger components and at limited positions on a component.
More recently, extruded condenser tubing has been provided with small voids in the extrusion profile of the tube which increase the surface area and increase turbulation of the refrigerant flowing therein. Extruded tubing is typically only used for high pressure environments (e.g. 1000 psi) such as in condensers because of the relatively high production costs compared to wrought products and the limitations on the alloy types suitable for extrusion.
Accordingly, a need remains for wrought products having features for enhancing heat transfer which may be used in a variety of heat exchanger components.
This need is met by the wrought product of the present invention having a micro-textured surface with textured features having dimensions of about 1-50 microns high, preferably about 20-40 microns high, about 1-200 microns wide, preferably about 20-50 microns wide and spaced apart by about 1-50 microns, preferably about 20-50 microns apart. The wrought product may be an aluminum brazing sheet or finstock used in concert with brazing sheet or be formed from stainless steel or copper/brass (Cu/brass). The aluminum may be a monolithic sheet of the AA series 1xxx, 2xxx, 3xxx, 4xxx, 5xxx, 6xxx, 7xxx, or 8xxx or a multilayer composite sheet with each layer being one of the AA series 1xxx, 2xxx, 3xxx, 4xxx, 5xxx, 6xxx, 7xxx, or 8xxx. The textured features are preferably roll textured into the aluminum sheet in a regular pattern of spaced apart cross hatches, spaced apart circles, dimples, parallel lines or combinations thereof. The roll texturing can be done as a finishing step by a material supplier or on a strip as a part of the fabrication practice for a particular component being made (e.g. roll textured while in a fin machine or tube making machine). The wrought sheet is about 25-1500 microns thick.
The wrought sheet of the present invention may be used to fabricate the tubing, fins or turbulators of a heat exchanger such as a radiator, oil cooler, heater, condenser, evaporator or the like. The textured aluminum surface may be present on one or both sides of the fins of a heat exchanger or on the surfaces of the heat exchanger exposed to the coolant and/or refrigerant (e.g. the internal surface of a radiator or heater tube).